A 90.degree. phase shifter circuit is used in, for example, a quadrature FM demodulator circuit to generate a signal 90.degree. different in phase from an input intermediate frequency (IF) signal. The 90.degree. phase shifter circuit of the FM demodulator circuit is formed of a BPF (band-pass filter) using LCR (coil, capacitor and resistor) as shown in FIG. 1, or a bi-quad LPF (low-pass filter) using an op-amp as shown in FIG. 2. With the BPF using the LCR, to vary the center frequency so as to provide a 90.degree. phase difference, the coil is adjusted, or a varicap diode is used to vary it to a capacitance value corresponding to its terminal voltage. With the by-quad LPF, a cut-off frequency or value of Q may be obtained by varying the constants of external elements connected to the op-amp.
However, even when either the BPF or bi-quad LPF is used as a 90.degree. phase shifter circuit, there is a problem that external components such as coils, capacitors, and resistors are required to form it as an IC (integrated circuit), so that its size and weight cannot be reduced, or the need for adjustment cannot be eliminated.
Accordingly, it is an object of the present invention to provide an active low-pass filter that is suitable as a 90.degree. phase shifter circuit, which requires few external components when formed as an IC.